As a material of a semiconductor device, silicon (Si) wafers have been widely used. Silicon wafers are acquired by growing silicon on the surface of silicon of the same type. Such a silicon wafer has excellent purity and crystallinity of a semiconductor integrated region and is advantageous in terms of yield of a semiconductor device and improvement of device characteristics.
In general, a silicon wafer is formed through a single crystal growth process to form a single crystal ingot, a slicing process to acquire a thin circular wafer by slicing the single crystal ingot, a grinding process to grind the outer circumferential part of the wafer acquired by the slicing process so as to prevent the wafer from being cracked and distorted, a lapping process to eliminate damage due to mechanical processing of the wafer, a polishing process to smooth the surface of the wafer, and a washing process to remove abrasives or particles from the wafer.
The process of polishing the surface of the wafer to eliminate damage generated in the preceding processes is necessary. A double sided polishing process of surfaces of the wafer is carried out by frictional force generated by relative movement between polishing pads attached to an upper surface plate and a lower surface plate and the wafer and the reaction of slurry containing polishing abrasives and various additives.
FIG. 1 is a view of a conventional wafer processing apparatus.
As shown in FIG. 1, the wafer processing apparatus includes a first pad 120 and a second pad 150 provided between a lower surface plate, i.e., a first surface plate 110, and an upper surface plate, i.e., a second surface plate 140. Carriers (not shown) into which wafers are inserted are placed between the respective pads 120 and 150, and are thus rotated between a first gear 170 and a second gear 180.
When slurry is injected under the condition that the carriers are placed between the first surface plate 110 and the second surface plate 140, both surfaces of the wafers within the carriers are polished through mechanical/chemical polishing.
The first surface plate 110 is rotated about a first rotary shaft 130 by a first motor 135, and the second surface plate 140 is rotated about a second rotary shaft 160 by a second motor 165. The first surface plate 110 and the second surface plate 140 are rotated in different directions, thus facilitating mechanical polishing of a surface of the wafer.
However, the conventional wafer processing apparatus has problems, as follows.
As the wafers are polished by rotation of the first pad and the second pad during the above-described process, the first pad and the second pad are worn. Here, the first pad and the second pad are not uniformly worn, but may be locally worn.
Further, the upper surface plate and the lower surface plate may be thermally expanded during polishing of a surface of the wafer. Here, a large amount of heat is generated from regions of the first pad and the second pad contacting the wafers, and thus the corresponding parts of the upper surface plate and the lower surface plate may be greatly thermally expanded.
FIG. 2 is a cross-sectional view of the first pad and the second pad of FIG. 1.
The parts of the upper surface plate and the lower surface plate corresponding to the regions of the first pad 120 and the second pad 150 contacting the wafers are thermally expanded, and the thickness of these regions of the first pad 120 and the second pad 150 contacting the wafers may be greater than other regions of the first pad 120 and the second pad 150, as shown in FIG. 2. Such local abrasion of the first pad 120 and the second pad 150 may cause non-uniform polishing surfaces of the wafers.
Further, when the upper surface plate and the lower surface plate are rotated about different rotary shafts, balance between the upper surface plate and the lower surface plate may be difficult, and when the upper surface plate and the lower surface plate are independently rotated under the condition that the first pad and the second pad is not uniformly worn, the upper surface plate and the lower surface plate are rotated so as not to be parallel with each other, thus causing severe non-uniform polishing surfaces of the wafers.